A major application of a DC inductor as a passive component is in a DC link of AC electrical drives. Inductors are used to reduce harmonics in the line currents in the input side rectifier system of an AC drive.
The use of permanent magnets in the DC inductors allows minimizing the cross-sectional area of the inductor core. The permanent magnets are arranged to the core structure in such a way that the magnetic flux or magnetization produced by the permanent magnets is opposite to that obtainable from the coil wound on the core structure. The opposing magnetization of coil and permanent magnets makes the resulting flux density smaller and enables thus smaller cross-sectional dimensions in the core to be used.
As is well known, permanent magnets have an ability to become demagnetized if an external magnetic field is applied to them. This external magnetic field has to be strong enough and applied opposite to the magnetization of the permanent magnet for permanent demagnetization. In the case of a DC inductor having a permanent magnet, demagnetization could occur if a considerably high current is led through the coil and/or if the structure of the core is not designed properly. The current that may cause demagnetization may be a result of a malfunction in the apparatus to which the DC inductor is connected.
Document EP 0 744 757 B1 discloses a DC reactor in which a permanent magnet is used and the above considerations are taken into account. The DC reactor in EP 0 744 757 B1 comprises a core structure to which the permanent magnets are attached. However, if very large currents flow through the coil winding during a fault, for example, the opposing magnetic field strength may be so large that permanent magnet is demagnetized permanently. Demagnetization of a permanent magnet in a DC inductor leads to a situation where the demagnetized piece has to be magnetized again. This means in practice that the DC inductor has to be removed from the apparatus and replaced with a new one.
One of the problems associated with the prior art structures relates thus to a permanent demagnetization of a permanent magnet in a DC inductor when excessive currents are flowing in the coil of the DC inductor.